Stepping up the pressure for political action on global warming, scientists for the second time in two months have called for strong measures to curb greenhouse-gas emissions.

The American Geophysical Union (AGU) – the world's largest scientific organization dedicated to Earth, atmospheric, and space sciences – warned Thursday that the world will need to reduce emissions by 50 percent below 1990 levels within this century if countries are serious about holding down warming to around 3.6 degrees F. by 2100.

Significantly warmer temperatures over that period would lead to seriously disruptive changes for societies as well as for ecosystems around the planet, many scientists say.

The warning comes on the heels of a similar call at the United Nation's global climate talks last month in Nusa Dua, Indonesia. There, scientists released the "Bali Declaration," which calls on negotiators to craft a new climate treaty that will aim to cut greenhouse-gas emissions by at least 50 percent below 1990 levels by 2050 to avoid what UN treaties refer to as "dangerous" human interference with climate.

Although the three major reports released last year by the UN's Intergovernmental Panel on Climate (IPCC) usually provide the scientific framework for policy debates on global warming, the AGU's statement is also likely to be influential, especially for those who distrust the UN, some analysts say.

The scientists' statement comes at a time when international climate talks led by both the UN and the US are moving into high gear.

Next week, the Bush administration is convening the second of it's Major Economies meetings in Hawaii in an attempt to bring big emitters from both industrial and developing countries together to find ways to deal with the climate problem. The administration says the results of its series of meetings, which is expected to end in July, would feed back into the formal UN process. For its part, the UN is moving forward on negotiations for a new climate treaty to replace the Kyoto Protocol, when its first commitment period ends in 2012.

Among many people, "there's a deep level of distrust toward the United Nations," says Matthew Nisbett, a professor of communications at American University in Washington who focuses on the intersection of science and public policy, adding that the IPCC's scientific reports are "routinely dismissed" by skeptics because of the panel's UN connection. However, when a prominent American scientific institution weighs in, whether it's the AGU or the National Academy of Sciences, "that shores up the credibility of the science."

In addition, he says, the statement needs to be seen in the larger context of increasing activism by some scientists. To be sure, individual scientists have tilted with the political process for decades, observes Gavin Schmidt, a climate researcher at NASA's Goddard Institute for Space Studies in New York. From the Manhattan Project to climate change, "some scientists have always been out there," he notes.

Still, Dr. Nisbett maintains that many scientists are giving fresh thought to their roles as communicators and contributors to policy. Indeed, the AGU's statement advocates a more active role for its members in communicating their work to the public and to people "who can implement policies to shape the future of climate."

This is a significant shift for the 50,000-member organization. "The AGU has not done this in the past," says AGU president Timothy Killeen.

Adds Nisbett, "You do see a mobilization of scientists ... around the idea of scientific activism." He notes, for example, that a group of prominent scientists formed Scientists and Engineers for America in 2006, to provide public education on science issues as well as train scientists to run for political office.

Another group, ScienceDebate 2008, aims to encourage this year's presidential candidates to include more science issues in their debates.